Interception computers for aircraft or the like



April 11, 1961 W. H. PENLEY ETAL INTERCEPTION COMPUTERS FOR AIRCRAFT ORTHE LIKE Filed Oct. 26, 1953 I FIG.1

s (Pf-w) T1 1 (Pi-MI b FIG.3

3 Sheets-Sheet 1 FIG.6

INA/EBITORS WILL/HM HENRY HENLEY CEUL (./OHN W YMqA/ BYE v W'IETORNE'YApril 11, 1961 w. H. PENLEY El'AL 2,979,262

INTERCEPTION CQMPUTERS FOR AIRCRAFT OR THE LIKE Filed Oct. 26, 1953 3Sheets-Sheet 2 ll-LIFIIV/ HENR NLEY 5cm HN WnYMqA/ BY 5 r I HT-roR NEYApril 1961 w. H. PENLEY ETAL 2,979,262

INTERCEPTION COMPUTERS FOR AIRCRAFT OR THE LIKE 3 Sheets-Sheet 5 FiledOct. 26, 1953 Y N R P T Y s N W IHM wd H; we

I441 YMaA/ .5 TT'OPN EY s 2,979,262 I Patented Apr. 11,1961

INTERCEPTION COMPUTERS FOR AIRCRAFT OR THE LIKE William Henry Penley, 6Western Close, Great Malvern, England, and Cecil John Wayman, 68 OldChurch Lane, Stanmore, England Filed Oct. 26, 1953, Ser. No. 388,358Claims priority, application Great Britain Oct. 31, 1952 3 Claims. (Cl.235187) The present invention relates to interception computers for usein conjunction with a system, for example a search radar system, forderiving information as to the presence and movement of other aircraftor the like in the vicinity of a given aircraft, the computer being usedto provide information for guiding the given aircraft on a manoeuvresuitable for intercepting one or a given group of others. Moreparticularly, but not exclusively, the invention relates to suchcomputers for use in an aircraft in conjunction with a search radarsystem mounted .in the aircraft.

It is an object of the present invention to provide an interceptioncomputer as set out above in which the type of interception manoeuvreemployed as a basis for comgputation is of novel and simple form, and inwhich the information is given simply, for example as a meter indicationshowing fly left, right or straight ahead,

In the accompanying drawings in which is shownone of the variouspossible embodiments Of'OllI' invention,

Figure 1 diagrammatically illustrates a large number of differentinterception manoeuvres; f

Figures 2 and 3 illustrate two specific manoeuvres, respectively, thesame being shown in greater detailv than in Figure 1 and with vectoranalyses;

Figures 4 and 5 show circuit diagrams of different parts of a computermechanism constructed in accordance with our invention; and

Figure 6 illustrates the derivation of certain equations which are usedin the computations effected by the mechanism shown in Figures 4 and 5.

In designing interception computers in accordance with the presentinvention, it was decided to adopt a standard type of interceptionmanoeuvre for the interceptor, which it is thought in certaincircumstances gives a number of advantages. In all cases it is assumedthat the course and speed of the target will remain constant from adatum time at which the interception computationvand manoeuvre isinitiated. It is also assumed that the interceptor is at approximatelythe same height as the target, having been directed to that end from theground.

The interception maneouvre adopted is illustrated in Figure l of theaccompanying drawings in which I represents the initial position of theinterceptor and T that of the target. The arrows show the directions offlight. To deal first with the end of the manoeuvre, it is arranged thatthe interceptor, which continuously flies at its maximum speed, turns onto the course of the target from one side or the other, with a turn ofits minimum turning radius flying in the same direction as the targetand at a predetermined distance, known in this specification as thein-behind distance, behind the target. From this position theinterception may be completed visually or using some form of lock-followradar system. Alternatively an attack with a guided missile may bedelivered from this point, as circumstances may require.

The rest of the manoeuvre before the final turn may be divided into twotypes, the actual type required being determined by the relativepositions of the target and the interceptor at the moment of initiationof the manoeuvre.

In the simplest type of manoeuvre, as illustrated by the paths 1 and 2in Figure 1, the interceptor flies down a straight path at its maximumspeed before making the final turn, and possibly makes a turn, again atits mini mum turning radius, before commencing the straight path, inorder to attain the required course as quickly as possible. In the othertype of manoeuvre, as illustrated by the paths 3 and 4, it is necessaryin effect for the interceptor to lose some time, and it flies on acircle of the minimum turning radius of the opposite curvature to thefinal turn, crossing the target course in so doing. In this case theinterceptor is initially too near the target course line to turn on toit with a turn even at its minimum turning radius. These two types ofinterception manoeuvre will be designated respectively the simple typeof pursuit interception manoeuvre and the crossing course type ofpursuit interception manoeuvre, and these terms where used in thisspecification, including the claims, are to be interpreted in accordancewith the explanation given in this paragraph. The mathematicalconditions governing the distinction between the two types of manoeuvreare given later in this specification.

According to the present invention an interception computer for guidinga given aircraft or the like (hereinafter called the interceptor) on oneof the two types of pursuit interception manoeuvre, namely the simpleand the crossing course types ,of,manoeuvre, toarrive at a predetermineddistancethereinafter called the in-behind distance) behind and on thesame course as one or a body of other aircraft orthe likej(hereinaftercalled the target) comprises means for deriving information as to theposition, course and speed of the target, means for deriving and/orsetting on information as to the position, course, speed and minimumturning radius of the interceptor, and the in-behind distance, meansresponsive to the initial relative positions and courses of the targetand the interceptor and the magnitude of the minimum turning radius ofthe interceptor for setting up the computer to operate for theappropriate one of the two types of pursuit interception manoeuvre,means for computing the time taken from a datum time for the interceptorto complete the appropriate type of interception manoeuvre, the initial.value of the course for the interception manoeuvre defined and utilisedin the computer being arbitrarily determined initially, means forcomputing the time taken from the same datum time, for the target to flyon its present course, which it is assumed to main-' tain, to the pointfurther along that course by the inbehind distance than the point atwhich the interceptor will fly on to it, means responsive to thedifference of the two computedtimes for continuously adjusting the valueof interception manoeuvre course in the computer to the correct one forwhich the time difference becomes zero, and means for indicating theinterception manoeuvre course defined in the computer.

The calculation to be carried out in the computer is of two differenttypes, corresponding to the two types of manoeuvre, although the basicprinciple in-each case is the same. The basic principle used is tocompute the two times required for the target and the interceptor toreach their interception positions as a function of (pf-M and thevarious constants of a given set of data where 1,11 is the correctinterceptor course to fly and is the target course. The difference ofthese times is fed to the input of a servo system, the motor of whichdrives a shaft through angles (pf-41 The system comes to rest when thetimes are equal, and (pg-w) has the value appropriate to a correctinterception. Knowing tp r, t is derived, and displayed or used as desired, for example as a correct heading marker on a P.P.I. display, as acourse correction angle bf-11;, t; being the actual heading, on a meter,or as a shaft rotation pf-p, fed to an auto-pilot.

The mathematical basis of the computations carried out will now begiven, followed by a description of one example of an electricalcomputer for solving the various equations. The mathematical basis willbe investigated for the two different types of interception manoeuvreseparately, Figures 2 and 3 respectively Showing diagrams illustratingthe investigation for the two types.

The simple type of pursuit interception manoeuvre is illustrated inFigure 2. I and T are the initial positions of the interceptor andtarget and V and V are their velocities. The computation is made withreference to a pair of orthogonal axes in the plane of the interception,having their origin at I the position of the interceptor, and one ofthem being parallel to the target course. The coordinates of the initialtarget position T are (a, b), the in-behind distance is c and theinterceptors minimum turning radius r. Figure 2 shows the interceptorflying on a course 1/, to make an interception, the angle between thetarget and interceptor courses therefore being equal to l/ -1,l Variousother points on the diagram POQQR and V are referred to below, theirsignificance being apparent from Figure 2.

The time taken for the interceptor to fly from I to Q along the courseindicated is given by:

Now

a- PR Bin (i ri r) IP Sin Wr BLT) When t -l =0, a correct interceptionwill be carried out. If t -t 0, the quantity (pf-V should be increased,i.e. the proposed interception course is to the right of that set, andconversely if t t O, the quantity bf-x11 should be decreased. In thecomputer the quantity V (t -t is fed to the input of a servo amplifierwhich controls a motor to drive a shaft through angles (a -w).Equilibrium is reached and the shaft comes to rest when the equation V(t t is satisfied, i.e. a correct interception will be carried out.

The simple type of manoeuvre worked out above, and shown in Figure 2 canonly be carried out if |a| r[1coS (rid-TH which is the conditionnecessary for the interceptor to be able to carry out just the requiredfinal turn on to the target course from its initial position. In theother cases, a crossing course type of manoeuvre as illustrated inFigure 3 may be carried out, in which the interceptor crosses the targetcourse during its flight to the interception point. The equations foracrossing course type of manoeuvre equivalent to Equations 1 and 2 abovewill now be worked out. The additional symbols used will be defined asthey arise, and are indicated in Figure 3. For a crossing course type ofmanoeuvre:

i;=%[arc IS-l-arc SP-l-arc PQ] (3) I= [(i'r)+ (Where .5 is the angle P65as shown in Fig. 3.) Similarly tT= tvWvT+ Wows 1 W le sin (1'T -b+2r sin5+0] (4) .5 may be evaluated as follows:

cos 5 where d: WS in Figure 3. But

d=1'-ar cos (ridl r) or 21- cos E=r+a+r cos (H- h") In the computer,Equation 5 is solved by a separate servo system in which a motor drivesa shaft through angles 5, the quantities 2rg and Zr sin of Equations 3and 4 being fed to the main servo system for solving the equationVr(IT-t1)=0 for a crossing course type of manoeuvre.

In the computer the two types of manoeuvre are distinguished and theequations solved correspondingly modified. Thus in the first case if [a|r[1cos (pf-w) the flight PI (Figure 2) will become negative. A diode isconnected in the circuit at a point such that if PI becomes negative,it, the diode, becomes conducting and keeps the output of that part ofthe computer at zero, until the condition is removed. Similarly 5, whichdepends only on a, (pi- 0 and r, would become negative, i.e. meaninglessif |a| r[lcos and an arrangement of diodes is used to limit the movementof the shaft of the 2 servo system to values in the range 0-90".

Finally it is convenient in the computer, to make (1 always positive, sothat it is not necessary to distinguish between attacks from port orstarboard of the target course. This removes the necessity for acomplicated switching system, all that is necessary being an arrangementwhereby iai is fed to the computing elements. This arrangement meansthat the values of (M-w) will also be limited to the range 0l80. Becauseof this some switching of the indicating system has to be carried outwhen a becomes negative.

A computer mechanism for solving the above problems will now bedescribed by way of example with reference to Figures 4 and 5 of theaccompanying drawings, the mechanism being for use in conjunction with aP.P.I. display in a pulse radar system.

Figure 4 shows a circuit diagram of parts of the computer for deriving anumber of quantities appearing in the equations to be solved, from theinformation available on the P.P.I. display of a search radar systemmounted in the interceptor. The P.P.I. display will be assumed to be onestabilised in azimuth and position, that is one in which true Northalways appears at a fixed direction on the C.R.T. screen, and thedisplay moves across the screen at a rate proportional to the speed ofthe interceptor, and on the same course.

The first requirement is a determination of the target course and speed,from the echo appearing on the display. To this end a target marker spotis produced on the display, the spot being positioned in dependence uponvoltages developed (as subsequently described) at terminals T14 and T15.The spot is initially aligned on the echo, and a switch S1 associatedwith the computer is closed. This operates a relay X, and anelectromagnetic clutch EC. The latter completes a mechanical drive froma constant speed clock motor CM to the shaft of a potentiometer P1, thetotal resistance of which is connected between a positive H.T. supplyterminal and earth. The voltage at the slider of the potentiometer P1 isapplied to the input of a high gain D.C. amplifier (the gain being ofthe order of a thousand), giving two outputs one positive and onenegative and balanced with respect to earth, which are applied acrossthe winding of a sine-cosine potentiometer SCPl. The amplifier A1 has aresistive feedback loop connected between its negative output and inputincluding a potentiometer P2 which is in series with a fixed resistorR1. Rotation of the shaft of the potentiometer P1 and movement of thetarget relative to the interceptor causes the echo and the target markerspot to separate. Sometime after closing the switch S1, for example onesecond later, the potentiometers P2 and SCPI are operated manually torealign the target marker spot on the echo. Thereafter it is assumedthat the target course and speed remain constant at the values sodetermined. The resulting positions of the sliders of the potentiometersP2 and SCPl are dependent upon the target speed V and target courserespectively and these quantities are utilized in the subsequentcomputation.

Since the clock motor CM drives the slider of potentiometer P1 atconstant speed, the voltage on the slider is proportional to time, and,the gain of the feedback loop for amplifier A1 being proportional to theoutput from the amplifier A1 is proportional toV x time, that is thedistance of the target from a datum position. By applying the outputacross the sine-cosine potentiometer SCPl, the shaft of which is rotatedby the L control, the outputs are obtained representing the co-ordinatesof the target with reference to a set of orthogonal axes parallel toNorth and East directions. Let these be (E N An alternative, and in someways improved, form of tracking unit for determining the target courseandspeed is described in application Serial No. 461,612 for ElectricalComputing Apparatus, filed October 11, 1954.

The co-ordinates of the intercepter are calculated with reference to thesame axes. At terminals T1 and T2 input voltages which are supplied, asby a generator 10, and which are proportional to the north and eastcomponents of the interceptors velocity are applied, these being N and EWhen relay X is unoperated the control of the output of the amplifiers Aand A is almost entirely dependent on the settings of potentiometers Pand P since resistors R and R are high in value compared with resistorsR R R and R When relay X is operated the inputs N; and E; are appliedthrough resistors R and R to the inputs of amplifiers A and A In thislatter condition the feedback loops of those amplifiers arecapacitative, so that the output voltages are proportional to the timeintegrals of N and B; respectively, that is N; and E the co-ordinates ofthe interceptor as required. Voltages representing the quantities N andE are fed to the inputs of the amplifiers A4 and A5 respectively.

As set out above the computations are made with respect to a set oforthogonal axes having their origin at the interceptor I, one of thembeing parallel to the target course. Referring to Figure 6, it will beseen that or I V i T'- I) sill Wr- T- 1) /"'1' The amplifier system A4,and A5 is provided for'cont putation of a and --b. The slider of thepotentiometer" SCPl is oriented so that the voltages on the output armsqrepresent +E and +N that is if V is the amplitude" of the output voltageof amplifier A, so that the voltages at the sliders are |V [(sin a land|V |(cos w). This is indicated in Figure 4 by the symbols +sin and +coson the relevant leads, and a similar notation is used throughout thedrawings on the various sine-cosine potentiometers' employed (sin andcos would indicate [V N-sin 1%) and ]V |(cos 1// In addition thequantities represented by voltages fed over the various leads areindicated against tliose leads. The input to the amplifier A4 isproportional to (N -N1); and the feedback loop is a simple resistiveone. s The outputiis applied across a sine-cosine potentiometer SCP2. V

Similarly an input (E -E is applied to theinput of amplifier A5 which isconnected in exactly the same way as amplifier A4, its output beingapplied to a sine-cosine potentiometer SCP3. The sliders of thepotentiometers being arranged as indicated in Figure 4, the output atterminal T3 is proportional to (N N cos P and' terminals T3 and T4 isproportional to;

(NT-NI) C05 P'r-( 'r r) Sin I T b from above. Similarly the sum of thevoltages at terminals T5 and T6 is proportional to:

Various other voltages for feeding to the main section of the computerare obtained in the part shown in Fig ure 4. A potentiometer P20 isconnected in a resistance chain across the positive H.T. supply andearth, and is set manually to give a voltage, at terminal T7,proportional to therequired in-behind distance 0. A potentiometer P3 isprovided, connected similarly, to enable the minimum turning radius r ofthe interceptor to be set in as a voltage at a terminal T8. The 111control is also used to rotate the rotor of a resolver synchro .RMl, forexample a magslip, across one of the stators of which an A.C. voltage isapplied by connection to terminal T9..

.from (6) The voltages at the unearthed ends of the rotor coils are arederived from the outputs of amplifiers A4and A5,

since, when V and 11 are correctly set, these are voltage proportionalrespectively to (N -N and (E -E the north and east components of thetargets displacement from the interceptor. These outputs are taken fromterminals T14 and T15 and fed to the target marker generator.

The switch S2 is part of a range change switch in the P.P.I., and isprovided to alter the resistance in the feedback paths of the amplifiersA4 and A5, so that. the voltage variations at terminals T12 and T13 arecorrectly scaled in relation to the ranges of the scan ap peering on theP.P.I. The voltages applied across the otentiometers SCP2 and SCP3 donot however vary. The resistors R2 are all of one value, and theresistors R3 of another value, to maintain balance in the circuits atthe two positions of the switch S2.

Provision is made for the initial setting of the target marker, beforeclosing the switch S1, by the potenti-- ometers P4 and P5 which are eachconnected between a positive and a negative H.T. supply terminal. Whenrelay X1 is not operated, contacts X1 and X3 earth the differentialfeedback paths of amplifiers A2 and A3, and connect the amplifier inputsto the sliders of potentiometers P4 and P5 and to a purely resistivefeedback: path. Voltages dependent upon the settings of potentiometersP4 and P are then fed from the outputs of amplifiers A2 and A3 to theinputs of amplifiers A4 and A5, and produce voltages at terminals T14and T15 which shift the target marker under the control of the settingsof the potentiometers P4 and P5. As soon as the switch S1 is closed andthe relay X operated, this no longer occurs.

The circuit diagram of the main servo system is shown in Figure 5 inwhich terminals T3-T8, T and T11 are connected to the same terminals inFigure 4. As stated previously the equation V (1 1 )=0 has to be solved,and this is done as a function of I' I For either type of manoeuvre, V(t t in the form f(-I --I is fed to the main servo amplifier A6, theoutput from which controls a servo motor generator M1-G1, which drives ashaft through angles equal to I l The generator G1 is included in knownmanner to provide velocity feedback to reduce hunting of the servosystem. The (hf-W shaft is used to rotate various potentiometers in theremainder of the computer which are included in circuits for computingthe various terms of f( I I' The system comes to rest therefore when f(I' I )=0, the shaft then being at an angle equal to the value of (Pf-Qfor a correct type of manoeuvre.

Considering the simple type of manoeuvre first, we

have for a solution I( I T) From Equations 1 and 2 for a solutionVoltages proportional to the various members of Equation 8 are computedseparately, and applied to the input of the amplifier A6. Following thisthrough in Figure 5, a voltage proportional to r is applied at terminalT8, and from this the circuit associated with amplifier A7 generatesvoltages proportional to the functions r, --r, r sin (I q r cos (V -Qetc. The voltage at terminal T8 is applied directly to the input of theamplifier A7, which has a resistive feedback loop. The output is appliedacross a sinecosine potentiometer SCP4, the shaft of which is rotated bythe I' I shaft driven by the servo motor M1. Outputs proportional tol-r, r, r sin I I and r cos (Pf-Q are taken off as indicated on thecircuit diagram. Voltages, whose sum is proportional to a, are appliedat terminals T5 and T6, and thence to the input of an amplifier A8. Theamplifier A8 has a resistive feedback loop and two balanced outputs, thepositive one of which is selected by a contact Y1, so that an output [a]is obtained irrespective of the sign of a. Contact Y1 is controlled by achangeover relay Y (as also are contacts Y2-Y5), the contacts Yl-YS eachtaking up one of two possible positions in dependence upon the polarityof the voltage applied to the relay Y. The input of the relay Y isconnected across balanced outputs from an amplifier A9 the input ofwhich is fed from one of the outputs of amplifier A8.

An amplifier A10 is fed at its input with voltages proportional to thequantities, lal, -r (one of the outputs from the circuit of theamplifier A7) and rcos I I (a further output from the circuit of theamplifier A7). Its outputs are connected across the winding of asine-cosine potentiometer SCPS, the sliders of which are driven by the(i i- P shaft. The feedback loop of the amplifier A10 is connected tothe sin slider of the potentiometer SCPS, so that the full output of theamplifier A10 is proportional to the function This quantity is the firstmember of Equation 8, and an output from the amplifier A10 is applied tothe input of the amplifier A6.

The term appears in the third member of Equation 8, and the l-cos sliderof potentiometer SCPS, at which a voltage proportional to this quantityappears, is connected to the input of an amplifier All the circuit ofwhich is used to derive a voltage proportional to the third member. Adiode V1 is connected between the negative output of the amplifier A10and the input, and becomes conducting if the voltage at the negativeoutput tries to become positive with respect to the input. This couldonly happen if the input voltage itself becomes negative, which occurswhen a is too small for the first type of manoeuvre to be carried out.This being so and the equations for the crossing course type ofmanoeuvre not including a term of the type derived by the circuit of theamplifier A10, the diode V1, becoming conducting, reduces the output ofthe amplifier A10 to zero, and voltages representing such terms are. notfed to the other parts of the computer. The diode V1 is thus included totake effect in the changeover from the one type of manoeuvre to theother, and in operation prevents meaningless negative quantities beingfed to other parts of the computer.

Other voltages are applied to the input of the amplifier A11,representing the quantities +r sin P I derived from the circuit of theamplifier A7, 0, applied at terminal T7, and b, applied at terminals T3and T4. There is one other input which is effective only in the crossingcourse type of manoeuvre and will not be discussed here. For the simpletype of manoeuvre therefore the input to the amplifier A11 isproportional to the quantity:

r+ cos ('l I' T) The feedback loop includes a variable resistancepotentiometer P6, which is varied in accordance with the target velocitysetting V which is made as described previously with reference to Figure4. The full output of the amplifier A11 is therefore proportional totimes the input. The output is applied across a potentiometer P7, whichis varied in accordance with the interceptor speed, which is set in as aconstant V equal to the maximum speed for level flight. The voltage atthe slider of the potentiometer P1 is therefore proportional to timesthe input, the minus sign arising as the negative output of theamplifier A11 is used. This voltage is applied to the input of the mainamplifier A6 and provides the third member of Equation 8.

The second member of Equation 8 is derived from a potentiometer P8,across which a voltage proportional to r is applied from the output ofamplifier A7. The shaft of the potentiometer P8 is driven by the (pf-pshaft so that the voltage at the slider is proportional to r(i,l/ Thisvoltage is applied to the input of the main amplifier A6, and providesthe second member of the Equation 8.

A second resolver synchro RMZ is provided, the rotor of which is drivenbythe (pf-111 shaft. The stators acted with alternating voltagesproportional to sin 1,0 and cos 1/1 applied at terminals T10 and T11respectively, through the contacts Y2 and Y3 which changeover the inputconnection to the synchro RMl as relay Y operates. The outputs acrossthe rotor windings and earthare alternating voltages proportional to sin1/1 and cos 1p which are applied to output terminals T16 and T17 throughcontacts Y4 and Y5 which reverse the connections as relay Y operates.The (cf-w) shaft actually rotates through angles since the servo systemcontrolling it is fed with |a|. Since the operator requires to know i/for comparison with \h, the changeover contacts Y2-Y5 have to beincluded in the input and output circuits of the synchro RM2, controlledby the relay Y in dependence upon the sign of a. The outputs fromterminals T16 and T17 are used to control a marker on the P.P.I. displayand to give an indication of the course to fly on an indicator 11.

The triode V2, is fed with a voltage proportional to r sin (yD;'- on itscathode, and r cos on its grid. Thus, when (pf-w) is approaching zero,the voltage on the control grid becomes increasingly positive withrespect to that on the cathode. The triode V2 then becomes heavilyconducting and the voltage at its anode drops, that is at the input ofthe main servo amplifier A6. This prevents the servo system running hardagainst the stop at the value (yb l )=0.

For a crossing-course type of manoeuvre, we have as before for aninterception:

rUr- 'r) which from Equations 3 and 4 gives Comparing this with Equation8 we see that the second members are the same in each case, and that,except for the last terms in them, the third members are the same.

The terms of Equation 8, which are left out, are those derived from theamplifier A10, and, as described previously, the diode V1 is provided tohold the voltages representing these terms at zero if the conditions fora crossing course interception hold. It remains therefore to compute theterms 2r and Zr sin and add these to the inputs to the appropriateamplifiers, that is A6 and A11 respectively, when, and only when, theconditions for a crossing course type of manoeuvre arise.

The quantity is given by Equation as follows:

A separate servo motor system is used to solve this equation, voltagesrepresenting the various terms of Equation 5 being fed in the correctsenses to the input of an amplifier A12, which controls a servo motorM2. The generator G2 is included in known manner to provide velocityfeedback and reduce hunting of the servo system. The motor M2 drives ashaft which comes to rest when, for a given set of values, the Equation5 is solved and the angular displacement of the shaft is 5.

Thus there are fed to the input of the amplifier A12 four voltagesrepresenting respectively the quantities r, obtained from an output ofthe amplifier A6, a, obtained from the output of the amplifier A8, r cos(il/fflp obtained from the cos slider of the potentiometer SCP4, and (2rcos 5), obtained from the cos slider of a sine-cosine potentiometerSCP6. The potentiometer SCP6 is connected across the +r and r outputsfrom the amplifier A7, and is driven by the g shaft controlled by theservo motor M2. The resistor R4 is made half the value of the otherresistors through which voltages are fed to the amplifier A12, toprovide the necessary scaling, and introduce the factor of two.

Thus thereis fed to the input of amplifier A12, a voltage representingthe function r+a+r cos l/ )2r,c0s 5 and as stated above, the {shaftrotates until a value is reached at which this function becomes zero.

Two outputs are required from the circuits. a voltage representing thequantity 2r sin 5, is taken from of a potentiometer P9, which is alsodriven by the 5 shaft. A voltage representing (+r) is applied across thepotentiometer P9 from the output of the amplifier A7. The voltagerepresenting 2r 5 is applied directly to the input of the mainservo-amplifier A6.

The only other provision to be made is that the outputs from the 5circuits are zero when the conditions for a crossing-course type ofmanoeuvre do not hold, i.e. when g would tend to become negative andhence meaningless. For this purpose a double diode V3 is included in thecircuit, the anode of one diode and the cathode of the other beingconnected together and to the input of the amplifier A12. The anode ofthe one diode is connected to the cos slider of potentiometer SCP6 andthe cathode of the other to the +sin slider of the same potentiometerSCP6, the voltages at these points representing -2r cos 5 and Zr sin 5respectively. As a result if the .cos 5 term tends to become positive,i.e. if 5 tends to exceed the one diode conducts and the current flowingin it efiects a voltage change at the input of amplifier A12 opposingany rotation of the shaft beyond the value =90. Similarly if .5 tends todecrease below 0, the other diode conducts, opposing any furtherrotation. In this way the possible values of g are held to the range0-90".

It will be appreciated that the computer described above with referenceto Figures 4 and 5 is one example of a mechanism which may be used tosolve the Equations 8 and 9, and that other types of computing mechanismmay equally well be employed. Similarly the parts of the mechanismdescribed with reference to Figure 4 for.

deriving the input information for the main computer circuits from theradar display may be varied.

We claim:

1. An interception computer for guiding an interceptor aircraft on oneof twotypes of pursuit interception manoeuvre, namely the simple and thecrossing course types of manoeuvre, to arrive at a predeterminedinbehind distance behind and on the same course as a target aircraft,said computer comprising means to supply information as to the initialposition of the target aircraft relative to the interceptor aircraft atthe beginning of a computation, means to supply information as to the 1course of the target aircraft, means to supply information as to thespeed of the target aircraft, means to supply information as to thespeed of the interceptor aircraft, means to supply information as to theminimum turning radius of the interceptor aircraft, means to supplyinformation as to the in-behind distance, means to compute from theinformation supplied by said information supplying means the course onwhich the interceptor aircraft should fly to effect the simple type ofinterception manoeuvre, means to compute from the information suppliedby said information supplying means the course on which the interceptoraircraft should fly to effect the crossing course type of manoeuvre,course indicating means, means to determine from information supplied,

One,

manoeuvre being selected if the simple type of interception manoeuvrewould have involved the interceptor aircraft in making aturn with aturning radius less than said turning radius, and means to supply tosaid course indicating means information as to the computed course ofthe interceptor aircraft selected by the last mentioned means.

2. An interception computer according to claim 1 in which the meanssupplying information as to the position of the target relative to theinterceptor comprises means for feeding in two voltages proportional inmagnitude to the components of the interceptors velocity in twodirections at right angles to one another, integrating means forderiving from the voltages fed in by the lastmentioncd means twovoltages which depend on the position of the the interceptor relative toa given set of axes, means for deriving two voltages which depend on theposition of the target relative to the said set of axes and means fordifferencing the voltages supplied by the said integrating means and thelast mentioned means to provide two voltages which are a measure of thein stantaneous position of the target relative to the in- ReferencesCited in the file of this patent UNITED STATES PATENTS 2,420,017 SandersMay 6, 1947 2,433,843 Hammond Jan. 6, 1948 2,476,746 Libman July 19,1949 2,600,159 Ergen June 10, 1952

